Process for producing TiAl intermetallic compound-base alloy materials having properties at high temperatures

ABSTRACT

A TiAl intermetallic compound-base alloy material having excellent strength properties at high temperatures and ductility, characterized by comprising: a fine alumina (Al 2  O 3 ) dispersed so as to give an O 2  concentration of 1000 to 5000 ppm by weight and in a particle diameter of 200 to 500 nm; a boride (TiB 2 ) dispersed to give a B concentration of 0.1 to 10 at % and in a particle diameter of not more than 500 nm; 1 to 3 at % of at least one of Cr, Mn, and V; and TiAl having a Ti content of 50 to 53 at % and an Al content of 47 to 50 at %, said TiAl intermetallic compound-base alloy material having been directly cast at a cooling rate of 10 3  to 10 5  ° C./sec and a process for producing the same. According to the present invention, exhaust valves for automobiles and materials for engine turbines for jet airplanes and the like having excellent tensile strength at high temperatures and ductility at high temperatures and room temperature are provided.

TECHNICAL FIELD

The present invention relates to TiAl intermetallic compound-base alloyshaving excellent tensile strength at high temperatures and ductility athigh temperatures and room temperature and a process for producing thesame.

BACKGROUND ART

A TiAl intermetallic compound-base alloy material is included amongmaterials under development for use as structural materials havingenvironmental resistance. Since this material has excellent strengthproperties at high temperatures, the development as a structuralmaterial for use at high temperatures in the future is expected in theart. Regarding this material, attention has been drawn to the strengthat high temperatures which is comparable to the property values of theconventional Ni-base and Co-base superalloys. Further, it should benoted that the specific gravity of the TiAl intermetallic compound-basealloy material is 3.8 while the specific gravity of the superalloys isclose to 10. When this fact is taken into consideration, the TiAlintermetallic compound-base alloy material is superior to thesuperalloys in specific strength at high temperatures. Therefore, it isa promissing material for advanced airplanes which should belightweight.

However, the upper limit of the service temperature of alloy materialshaving strength at high temperatures, such as these superalloys,including TiAl intermetallic compound-base alloy materials is 900° C.,and metallic materials having satisfactory strength at 900 to 1100° C.have not been developed in the art. Rather, nonmetallic materials, suchas ceramics and C/C (carbon/carbon fibers), are used in temperatures of1000° C. or above. These nonmetallic materials has high strength at hightemperatures. However, fracture, in most cases, is created withinelastic stress, so that the ductility is zero. For this reason, thedevelopment of alloy materials having ductility has been desired fromthe viewpoint of safety.

In the prior art, there is a near net shape casting technique where anintermetallic compound sheet is produced. The technique for producing anear net shape sheet has been rapidly advanced in recent years. Inparticular, in the metallic materials, the advance in the application tothe production of stainless steel sheets is significant. Various castingprocess have been proposed for the production of the sheet. Among them,a twin roll process is suitable for the production of a continuous sheethaving an even thickness.

A nickel-aluminum intermetallic compound (Ni₃ Al) having ductilityimproved by the addition of a very small amount of boron is known as anexample of the application of the above technique to intermetalliccompound materials. This has been reported in an internationalconference concerning "Casting of Near Net Shape Products" held inNovember 1988 (Proceeding of an International Symposium of Near NetShape Products, pp. 315-333, issued by The Metallurgical Society).Further, a process for producing a TiAl intermetallic compound sheet isdescribed in Japanese Patent Application No. 501367/1991.

Further, for TiAl intermetallic compound materials with boron addedthereto, U.S. Pat. No. 4,842,820 discloses a production processutilizing plasma melt process and isothermal forging, and U.S. Pat. No.4,751,048 discloses a production process utilizing mechanical alloying.

For TiAl intermetallic compound materials with chromium added thereto,U.S. Pat. Nos. 4,842,819 and 4,879,092 disclose a production processutilizing plasma melt process and isothermal forging.

DISCLOSURE OF INVENTION

An object of the present invention is to provide a TiAl-baseintermetallic compound material having enhanced strength properties athigh temperatures and, at the same time, to further improve theductility at room temperature while maintaining the strength propertiesat high temperatures.

According to one aspect of the present invention, there is provided aTiAl intermetallic compound-base alloy material having excellentstrength properties at high temperatures, characterized by comprising: afine alumina (Al₂ O₃) dispersed to give an O₂ concentration of 1000 to5000 ppm by weight and in a particle diameter of 200 to 500 nm; and TiAlhaving a Ti content of 50 to 53 at % and an Al content of 47 to 50 at %,or further comprising at least one of Cr, Mn, and V in an amount of 1 to3 at % and TiAl.

According to another aspect of the present invention, there is provideda TiAl intermetallic compound-base alloy material having excellentstrength properties at high temperatures and ductility, characterized bycomprising: a dispersed fine alumina (Al₂ O₃); and a boride (TiB₂)dispersed to give a B concentration of 0.1 to 10 at % and in a particlediameter of not more than 500 nm; or further comprising 1 to 3 at % ofat least one of Cr, Mn, and V; more preferably, said TiAl intermetalliccompound-base alloy material having been directly cast at a cooling rateof 10³ to 10⁵ ° C./sec.

According to a further aspect of the present invention, there isprovided a process for producing a TiAl intermetallic compound-basealloy material having excellent strength and ductility at roomtemperature and high temperatures, characterized in that a fine TiB₂having a diameter of not more than 500 nm is previously dispersed in thepreparation of a master alloy by a melt process, a calcia crucible or analumina (Al₂ O₃) crucible coated with a calcia (CaO) powder is used as acrucible in high-frequency melting in the melting of the master alloy, aTi plate is heated to 800 to 1100° C. in a VIM (vacuum inductionmelting) vessel to conduct gettering of oxygen present within thevessel, thereby lowering the concentration of oxygen in the atmosphereto not more than 0.2%, casting is carried out in this state to producean ingot, and the ingot is ground and used as a raw material formechanical alloying to prepare a powder which is then sintered andmolded, or alternatively the ingot is subjected to isothermal forging tobring the structure to a fine grain structure.

According to a further aspect of the present invention, there isprovided a process for producing a TiAl intermetallic compound-basealloy foil, characterized in that the TiAl intermetallic compound-basealloy sheet produced by the above process is molded by high-temperaturepack rolling wherein an alumina foil or a calcia powder is used as arelease material and packing is carried out using a Ti alloy orstainless steel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional side view of a direct castingmachine, for producing a sheet, used in practicing the presentinvention;

FIG. 2 is a photograph showing the crystal structure of alumina in amaterial (oxygen concentration 1.5 wt %) according to the presentinvention;

FIG. 3 is an enlarged view of the photograph shown in FIG. 2;

FIG. 4 is a photograph showing the crystal structure of alumina in amaterial (oxygen concentration 0.25 wt. %) according to the presentinvention;

FIG. 5 is an enlarged view of the photograph shown in FIG. 4;

FIG. 6 is a graph showing the relationship between the oxygenconcentration (ppm) and the tensile strength (MPa) at 1000° C. for aTi-47Al-3Cr material (at %) which is a material of the presentinvention;

FIG. 7 is a graph showing a comparison between the specific strength ofa Ti-47Al-3Cr material, a material of the present invention, and that ofpure Ti, a Ti alloy, and a conventional TiAl alloy;

FIG. 8 is a metallographic photograph in section, in the thicknesswisedirection, of a TiAl intermetallic compound-base alloy material withTiB₂ not added thereto;

FIG. 9 is a metallographic photograph in section, in the thicknesswisedirection, of a TiAl intermetallic compound-base alloy material with 0.1at % TiB₂ added thereto;

FIG. 10 is a metallographic photograph in section, in the thicknesswisedirection, of a TiAl intermetallic compound-base alloy material with 1at % TiB₂ added thereto;

FIG. 11 is a graph showing the tensile strength of TiAl intermetalliccompound-base alloy materials, with the amount of TiB₂ added beingvaried (0, 0.1 and 1 at %), produced by direct casting;

FIG. 12 is a graph showing the ductility of TiAl intermetalliccompound-base alloy materials, with the amount of TiB₂ added beingvaried (0, 0.1 and 1 at %), produced by direct casting;

FIG. 13 is a graph showing a comparison between the yieldstress/temperature properties of a Ti-50Al-0.1TiB₂ sheet produced by theprocess of the present invention (twin-roll casting, oxygenconcentration 2500 ppm) and that produced by the conventional process(VIM melt process, isothermal forging, oxygen concentration 1000 ppm);

FIG. 14 is a graph showing a comparison between the yieldstress/temperature properties of a Ti-50Al-1TiB2 sheet produced by theprocess of the present invention (twin-roll casting, oxygenconcentration 2500 ppm) and that produced by the conventional process(VIM melt process, isothermal forging, oxygen concentration 1000 ppm);

FIG. 15 is a graph showing the comparison between the properties of aTi--Al intermetallic compound-base alloy material of the presentinvention and those of a conventional Ti--Al material; and

FIG. 16 is a graph showing the comparison between the properties of aTi--Al intermetallic compound-base alloy material of the presentinvention and those of superalloys.

BEST MODE FOR CARRYING OUT THE INVENTION

The present inventors have found that, in the matrix of the conventionalTiAl intermetallic compound-base alloy, high strength at hightemperatures can be provided by dispersing a fine oxide Al₂ O₃ having aparticle diameter of not more than 500 nm at intervals of not more than10 μm in the matrix.

Further, the present inventors have found that the dispersion of aboride (TiB₂) in combination with the fine oxide in the matrix of theTiAl intermetallic compound-base alloy enables the ductility at roomtemperature to be ensured while maintaining the high strength at hightemperatures attained by the dispersion of Al₂ O₃ alone.

Specifically, despite the improvement in ductility at room temperatureby refinement, a marked improvement in strength at high temperaturescould be attained without a lowering in strength caused by promotion ofgrain boundary sliding expected by grain refining at high temperatures.

The mechanism through which the strength at high temperatures isdeveloped will be described.

For alloy materials, the strength development temperature region isgenerally recognized to be up to a temperature obtained by multiplyingthe melting point, in terms of absolute temperature, of the material by0.6. In temperatures above this value, the diffusion becomes dominant,and the material is viscously deformed at a lower stress than the yieldpoint. That is, the deformation is mainly creep deformation.

In the case of binary system, TiAl has a melting point of 1470° C.Therefore, the melting point in terms of absolute temperature is 1743K,and the value obtained by multiplying this absolute temperature value by0.6 is 1046K. That is, the upper limit of the strength developmenttemperature is 772° C. when the temperature is above 772° C., grainboundary sliding and dislocation slip are activated, resulting inlowered yield stress and lowered strength. For TiAl intermetalliccompound-base alloys, which have been subjected to conventional heattreatment or isothermal forging, the strength was 180 to 300 MPa at 800°C., 80 to 150 MPa at 1000° C., and not more than 40 MPa at 1100° C.

By contrast, the material, of the present invention, having a matrixwith a fine oxide Al₂ O₃ being dispersed alone or in combination withTiB₂ has improved yield stress and strength. The reason for this isconsidered as follows.

At the outset, regarding the grain boundary sliding, the presence of thefine oxide in the grain boundaries increases the stability of grains athigh temperatures and causes the grain boundaries to be pinned,resulting in improved strength. On the other hand, in the case oftransgranular sliding, the dislocation is accumulated in thetransgranular fine oxide, inhibiting further movement caused bydislocation, resulting in increased strength. Further, since thepropagation of dislocation is inhibited in this way, an intensiveincrease in dislocation density, which is the drive force forrecrystallization, is reduced preventing a lowering in strength causedby dynamic recrystallization. These mechanisms can be explained by thefact that Al₂ O₃ fine oxide dispersed alone or Al₂ O₃ fine oxide andTiB₂ dispersed in combination can in itself function as fine inclusions.

Another important mechanism is strengthening by dissolution of oxygen ina solid solution form in an α₂ phase constituting the lamellar structureof TiAl. The α₂ phase functions as a getter of oxygen in TiAl. In thelamellar structure, in TiAl, constituted by γ phase and α₂ phase, thespace between phases is small particularly in α₂ in the second phase andabout 10 nm. For this reason, the determination of the concentration ofoxygen contained in this phase has been difficult. However, thequantification of the oxygen concentration has become possible by usinga high-oxygen (1.5 wt %) material, facilitating the identification ofoxygen, and using as analyzing means AP-FIM analysis which enables thedetermination of area and the quantitative analysis on the order ofatom. As a result, it has been found that most of the oxygen is absorbedin the α₂ phase, and the amount of the oxygen in a solid solution formin the α₂ phase is on a level of 5 at % (R. Uemori, T. Hanamura and H.Morikawa, Scripta Metall. Mater., 26, 1992, 969).

Thereafter, a confirmative experiment using AP-FIM was conducted by aFrench researcher, making it possible to perform the determination ofoxygen concentration on a ppm level (A. Huguet and A. Menand, AppliedScience, 76/77, 1994, 191). According to this determination, the upperlimit of the amount of oxygen dissolved in a solid solution form in theγ phase is 300 wt. ppm, and most of the oxygen is dissolved in a solidsolution form in the α₂ phase. The upper limit of the amount of oxygendissolved in a solid solution form in the α₂ phase is estimated to be 5at %.

From the above facts, it is considered that oxygen other than that inAl₂ O₃ is present in the α₂ phase and contributes to solid solutionstrengthening of the α₂ phase. Further, in a Ti alloy, oxygen isgenerally known as an α phase stabilizing element. Therefore, it isconsidered that the dissolution of oxygen in the α₂ phase enhances thestability of the α2 phase at high temperatures, contributing to thestrength at high temperatures. This leads to the inhibition of dynamicrecrystallization.

As described above, it is considered that the strength at hightemperatures can be attained by synergistic effect of the inhibition ofgrain boundary sliding, the inhibition of transgranular sliding, and theinhibition of dynamic recrystallization.

The mechanism for fine particle dispersion strengthening attained by thefine oxide alone will be discussed using the Ashby's equation.

At the outset, parameters are set as follows.

R: Radius of fine oxide

μ: Elastic constant of γTiAl

l: Average distance between particles of fine oxide

f: Volume fraction of fine oxide

b: Burgers vector of dislocation in γTiAl

M: Poisson constant

σ: Strength (stress)

τ: Strength (shear stress)

When the volume of the fine oxide is first taken into consideration, thefollowing relational expression is established. ##EQU1## Thus, ##EQU2##The shear stress can be expressed as follows. ##EQU3## Therefore, theshear stress created by dispersion of the fine oxide is: ##EQU4## Fromthis, the strength (stress) by the dispersion of the fine oxide is:##EQU5##

Next, the parameters are set at the following respective values.

M=3, μ=5×10¹⁰ Pa, b=0.25 nm, R=200 nm, and f =0.1% to 10⁻³.

The substitution of these values for the equation (6) gives: ##EQU6##

Thus, according to the estimation from he Ashby logical expression, itis concluded that the increase in strength by contribution of thedispersion of the fine oxide alone is very small, i.e., on a level of 4MPa at the largest. From the results of the calculation, it can be saidthat the reason why the actual high-temperature strength at 1100° C.could be improved from the conventional level 40 MPa to the level of 220MPa cannot be satisfactorily explained by mere dispersion of the fineoxidation alone. After all, this improvement is considered to beattained by the synergistic effect of the dispersion of the fine oxide,the solid solution strengthening of oxygen in the α₂ phase in the finelamella, and the matching between the lamella and the fine oxide. In thesynergistic effect, it is considered that, in particular, a smallerdistance between fine lamellae and, further, a better coincidencebetween the size of the fine oxide and the space between fine lamellaeoffer higher contribution of lamella interface matching, resulting inhigher strength at high temperatures.

The increase in strength by the reduction in the space between thelamellae is supported, for example, by the results of an experimentusing TiAl polysynthetic-twin results conducted by Umakoshi et al. (Y.Umakoshi, T. Nakano and T. Yamane, Mat. Sci. and Eng., A152, 1992, 81).This article substantiates that the space between lamellae exhibits HallPetch's relation with respect to the strength.

In the present invention, in order to attain the strength at hightemperatures, the particle diameter of finely dispersed Al₂ O₃ should benot more than 500 mn. The lower limit of the particle diameter is 100nm, preferably 200 nm. This is because when the particle diameter isless than 100 nm, the interaction between the particles and thedislocation is so small that the inhibition of transgranular sliding orthe like is unsatisfactory. On the other hand, when the particlediameter is excessively large and exceeds 500 nm, the particles serve asthe origin of cracking, resulting in deteriorated ductility.

The oxygen concentration for providing Al₂ O₃ having the above particlediameter range is 1000 to 5000 ppm by weight, preferably 1000 to 4000ppm by weight, more preferably 1000 to 2500 ppm by weight. The reasonfor this is as follows. When the oxygen concentration is less than 1000ppm by weight, the amount of the oxide is so small that no satisfactorystrength at high temperatures can be provided. On the other hand, whenit exceeds 5000 ppm by weight, the Al₂ O₃ is coarsened and functions asthe origin of cracking, resulting in deteriorated ductility andtoughness.

At least one of Cr, Mn, and V may be added, as an additive element, in atotal amount of not more than 3 at % to the TiAl intermetalliccompound-base alloy material according to the present invention. Cr, Mn,and V, when isothermal forging of the material is added, function torefine the structure and to precipitate a β phase in γ grain boundaries,enhancing superplasticity at high temperatures and thus improving theductility at high temperatures.

In general, the refinement of the structure can improve the strength andductility at room temperature. In this case, however, fine grainspromote grain boundary sliding, resulting in deteriorated strength athigh temperatures. In the material of the present invention, however, byvirtue of the dispersion of the fine oxide and the addition of TiB₂, theductility at high temperatures and the ductility at room temperature canbe improved by 2% or more without rapidly deteriorating the strength athigh temperatures.

Uniform refinement of the structure is important to the enhancement ofthe ductility at room temperature of the TiAl intermetalliccompound-base alloy material. For attaining this purpose, isothermalforging at a low strain rate in a high temperature region around 1200°C. has been necessary in the prior art. In a usual casting structure,for example, in a two-component system having a composition of Ti: 50 at% and Al: 50 at %, grains in the as-cast structure are greatly coarsenedto a diameter of about 2 mm. Further, a TiAl intermetallic compound-basealloy sheet produced by direct sheet casting has a problem that thethickness of the as-cast material is as small as 1.5 mm, making itimpossible to conduct pressing with a reduction ratio of about 80% byisothermal forging which is necessary for the regulation of thestructure.

By contrast, according to the present invention, even fine grains havinga diameter of 20 μm could be provided in the as-cast state. Themechanism through which the ductility at room temperature is developedwill be described.

Fine grains of alumina first functions as a nucleation site for therefinement of grains. When alumina is used alone, the growth of grainswith the alumina functioning as the nucleation site is not easy. Forthis reason, another precipitation phase, which, together with thegrains, precipitates on the alumina is used. Preferably, thisprecipitation phase is present in a melted state in the molten TiAlintermetallic compound-base alloy and precipitates in a matched manneron the alumina upon solidification of the TiAl intermetalliccompound-base alloy. The precipitation reduces elements constituting theprecipitate around the alumina. This shortens the latent period fornucleation in a localized area around the alumina, creating nucleationof grains. When a large number of alumina particles are finely dispersedin advance, the nucleation of grains occurs simultaneously in manyareas, creating a structure of fine grains.

An experiment was conducted using TiB₂ as a precipitate which matchesboth the alumina and the γTiAl phase. As a result, a structure ofuniform fine grains having a size of 100 μm could be provided by theconventional high frequency melting (VIM melting) and the conventionalingot casting.

The combined use of the above effect and supercooling effect is moreeffective in attaining further refinement of the structure. Rapidcooling may be utilized for this purpose. Twin-roll direct casting of aTiAl master alloy with TiB₂ dispersed therein enabled the grain diameterto be regulated to 20 μm. By virtue of the fine uniform structure, atensile ductility at room temperature of 2.12% could be provided bytwin-roll casting (without any subsequent treatment) +HIP (hightemperature isostatic pressure) treatment +stress relieving annealing at1000° C.

According to the present invention, in the melting of the alloy, analumina (Al₂ O₃) crucible coated with a calcia (CaO) powder is used asthe crucible used in the high frequency melting to prevent inclusion ofcontaminants, such as oxygen, from the crucible. This is because thethermodynamic stability of calcia is better than that of alumina. Morespecifically, alumina is reduced by a reaction with Ti as a rawmaterial, whereas calcia hardly reacts with Ti.

In the present invention, a Ti plate is heated to around 1000° C. withina vessel in a twin-roll casting system to conduct gettering of oxygenwithin the vessel, thereby lowering the concentration of oxygen in theatmosphere. In this case, the oxygen concentration of the atmosphereshould be lowered to not more than 200 ppm. Preferably, it is lowered tonot more than 100 ppm. The lowering in the oxygen concentration of theatmosphere to not more than 100 ppm in combination with the coating ofthe crucible enables the concentration of oxygen in the cast material tobe directly lowered to not more than 0.25% by weight. This offersoptimal control of the alumina oxide in the matrix. Even when theconcentration of oxygen in the atmosphere is not more than 200 ppm, theoxide can be controlled.

In the present invention, the alumina oxide can be dispersed as fineparticles having a diameter of not more than 500 nm. This is because themolten metal is always in the state of agitation by strong convectioncreated by high frequency during melting of the parent material by highfrequency melting and this state is frozen by twin-roll direct casting.In this case, freezing refers to a solidified state with the dispersionof the oxide at high temperatures being maintained. This can inhibitcoarsening caused by aggregation of the oxide.

In general, the alumina oxide is likely to be coarsened by aggregation.In the case of oxygen concentration 1.5 wt %, in the state of a usualcast ingot, the alumina oxide is unfavorably coarsened to a diameter of50 μm. Freezing by means of a twin roll alone leads to coarsening to adiameter of 2 to 3 μm. By contrast, according to the present invention,the combination of the low oxygen concentration 0.25 wt % with the twinroll process can realize dispersion of fine oxide particles having adiameter of not more than 500 nm.

In the present invention, the addition of TiB₂ necessary for therefinement of the structure is performed by introducing the TiB₂ wrappedin a Ti foil into a molten metal 2 to 3 min before pouring into a moldin the preparation of a master alloy by the VIM process. This shortensthe residence time in the molten metal, preventing the aggregation ofTiB₂.

EXAMPLES

The present invention will be described with reference to the followingexamples and comparative examples.

At the outset, one embodiment of a machine for the production of a sheetby direct casting used in the present invention will be described withreference to FIG. 1. In FIG. 1, a tundish 2 for evenly supplying amolten metal is disposed below a crucible 1 for melting an intermetalliccompound, and a pouring basin 5 (mold) constituted by a cooling drum 3and a side dam 4 is provided just under the tundish 2. All of them areprovided within an atmosphere controller 7. Numeral 8 designates aninert gas introduction mechanism, and numeral 9 designates an exhaustmechanism.

EXAMPLE 1

An aluminum metal and a sponge titanium were mixed together to give acomposition of Ti: 50 at % and Al: 50 at %, and the mixture was meltedin an alumina (Al₂ O₃) crucible coated with a calcia (CaO) powder byhigh-frequency melting (VIM), whereby a master alloy was prepared by amelt process. The crucible had a size of 110 (inner diameter)×125 (outerdiameter)×180 (height) mm. The high-frequency melting was conductedunder conditions of voltage: 62 V, current: 76 A, and power: 10 kW forthe first 10 min and then under conditions of voltage: 75 V, current: 80A, and power: 20 kW for the last 20 min.

The master alloy was taken off in a weight range of from 2000 to 3500 gand placed in the above crucible. The vessel was hermetically sealed andevacuated, and the atmosphere was replaced with Ar gas.

In the Ar atmosphere, a Ti plate was heated to around 1000° C. in avessel of a twin-roll casting system to conduct gettering of oxygenpresent within the vessel, thereby lowering the oxygen concentration ofthe atmosphere. In this case, the concentration of oxygen in the vesselof the twin-roll casting system was continuously monitored with anoxygen analyzer. The oxygen concentration before gettering was 1%,whereas the gettering could lowered the oxygen concentration to not morethan 0.2%. Heating of the getter at 1000° C. was continued until theproduction of the twin-roll sheet was completed.

The master alloy was then heat-melted in the above Ar atmosphere andadjusted to a temperature of 1600° C. and poured into a pouring basin 5through a tundish with an opening having a width of 4 mm and a length of95 mm. The cooling drum 3 constituting the pouring basin 5 comprised apair of drums which had a diameter of 300 mm and a width of 100 mm andwere made of a copper alloy and internally cooled. The molten metal wasrapidly solidified at a cooling rate of 10³ ° C./sec with the drumsupporting force being kept constant, thereby preparing a continuouscast strip in a sheet form. In this case, the sheet thickness wasregulated to 1.5 mm by setting the twin roll nip (1.5 mm), therotational speed of rolls (0.44 m/sec), and the temperature of themolten metal at the time of tilting thereof (1600° C.).

The cast strip 6 delivered from the cooling drum 3 was inserted into acarrier vessel while gradual cooling at a rate of 1° C./sec within theatmosphere control vessel 7.

In the structure in section, in the direction of casting, of the caststrip thus obtained, the as-cast solidification structure comprised onlya columnar crystal extending from both surfaces of the cast strip towardthe center portion thereof or comprised a mixture of the above columnarcrystal with an equiaxed crystal present around the center portion ofthe cast strip.

As shown in FIGS. 2 to 5, in the microstructure of the cast stripprepared according to the present invention, Al₂ O₃ having a diameter ofnot more than 500 nm is finely dispersed. FIG. 2 shows the crystalstructure of alumina in a material (oxygen concentration 1.5 wt %). FIG.3 is an enlarged view of the crystal structure shown in FIG. 2. FIG. 4shows the crystal structure of alumina in a material (oxygenconcentration 0.25 wt %), and FIG. 5 is an enlarged view of the crystalstructure shown in FIG. 4.

EXAMPLE 2

An aluminum metal and a sponge titanium were melted in an alumina (Al₂O₃) crucible coated with a calcia (CaO) powder by high-frequency melting(VIM), and Cr was added as an additive element. Thus, a master alloy ofTi-47Al-3Cr was prepared by a melt process. The procedure of Example 1was then repeated. That is, twin-roll casting of a sheet was carriedout, and a cast strip was prepared therefrom by high-temperature hotisostatic pressing (HIP).

Mechanical properties at high temperatures of the cast strip thusobtained are tabulated in Table 1. The test on the mechanical propertieswere carried out under conditions of vacuum of test atmosphere 5×10⁻⁵Torr and strain rate 7.2×10⁻⁴ sec⁻¹.

                  TABLE 1                                                         ______________________________________                                        High-temp. tensile properties                                                 (Example 2)                                                                             Yield        Tensile                                                Test temp.                                                                              stress       strength                                                                              Ductility                                      (° C.)                                                                           (MPa)        (MPa)   (%)                                            ______________________________________                                        800       389          579     13.2                                           1000      316          385     26.5                                           1100      179          226     49.3                                           ______________________________________                                    

The influence of the oxygen concentration on the tensile strength at1000° C. for the Ti-47Al-3Cr material is shown in FIG. 6. As can be seenfrom FIG. 6, the upper limit of the oxygen concentration in the presentinvention is not more than 5000 ppm by weight, preferably not more than4000 ppm by weight.

COMPARATIVE EXAMPLES 1 to 5

For comparison, conventional TiAl alloy materials as shown in Table 2,that is, a plasma-melted, annealed material, an isothermally formedmaterial, and commercially available superalloys were provided.

                  TABLE 2                                                         ______________________________________                                        Composition and working and heat treatment conditions                         for samples                                                                           Composition                                                                   (at %)   Working and heat treatment                                   ______________________________________                                        Example                                                                              1      Ti-50Al    Twin-roll casting of sheet + HIP                            2      Ti-47Al-3Cr                                                                              Twin-roll casting of sheet + HIP                            1      Ti-47Al-3Cr                                                                              Plasma melting + annealing                                  2      Ti-47Al-3Cr                                                                              Plasma melting + isothermal forging                  Comp. Ex.                                                                            3      Ti-50Al    Plasma melting + isothermal forging                         4      Co-base    Commercially available material                                    superalloy                                                             5      Ni-base    Commercially available material                                    superalloy                                                      ______________________________________                                    

Next, the high-temperature properties of the TiAl intermetalliccompound-base alloy material according to the present invention werecompared with those of the conventional material. For both materials,the relationship between the specific strength and the temperature isshown in FIG. 7. From data shown in FIG. 7, it can be said that thematerial of the present invention has the highest strength as the alloysystem in the high-temperature specific strength properties.

According to the present invention, the cast strip and the treated sheethad markedly improved mechanical properties. This improvement isconsidered to derive from the fact that a reduction in inclusion ofimpurities resulted in a reduction in oxygen concentration of thematerial, offering an optimal oxygen content. Further, agitation by highfrequency finely divided the oxide, and this state could be frozen bydirect twin-roll casting.

EXAMPLES 3 to 5

An aluminum metal and a sponge titanium were mixed together to give acomposition of Ti: 50 at % and Al: 50 at %, and the mixture was meltedby high-frequency melting (VIM), whereby a master alloy was prepared bya melt process. In this case, in the final stage of pouring of themolten metal, a TiB₂ powder, in an amount corresponding to 1 at %,wrapped in an Al foil was added to the molten metal, thereby dispersingthe TiB₂ powder in the γTiAl matrix.

Then, an alumina (Al₂ O₃) crucible coated with a calcia (CaO) powder wasprovided as a crucible used in high-frequency melting before melting.

The master alloy was taken off in a weight range of from 2000 to 3500 gand placed in the above crucible. The vessel was hermetically sealed andevacuated, and the atmosphere was replaced with Ar gas.

In the Ar atmosphere, a Ti plate was heated to around 1000° C. in avessel of a twin-roll casting system, and oxygen present within thevessel was subjected to gettering to lower the oxygen concentration ofthe atmosphere. In this case, the concentration of oxygen in the vesselof the twin-roll casting system was continuously monitored with anoxygen analyzer. The oxygen concentration before gettering was 1%,whereas the gettering could lowered the oxygen concentration to not morethan 0.2%.

Heating of the getter at 1000° C. was continued until the production ofthe twin-roll sheet was completed.

The master alloy was then heat-melted in the above Ar atmosphere,adjusted to a temperature of 1700° C. and poured into a pouring basin 5through a tundish with an opening having a width of 4 mm and a length of95 mm. The cooling drum 3 constituting the pouring basin 5 comprised apair of drums which had a diameter of 300 mm and a width of 100 mm andwere made of a copper alloy and internally cooled. The molten metal wasrapidly solidified at a cooling rate of 10³ ° C./sec with the drumsupporting force being kept constant, thereby preparing a continuouscast strip in a sheet form. In this case, the sheet thickness wasregulated to 1.5 mm by setting the twin roll nip (1.5 mm), therotational speed of rolls (0.44 m/sec), and the temperature of themolten metal at the time of tilting thereof (1600° C.).

The cast strip 6 delivered from the cooling drum 3 was inserted into acarrier vessel while gradual cooling at a rate of 1° C./sec within theatmosphere control vessel 7.

Compositions of samples used as examples and working and heat treatmentconditions are tabulated in Table 3.

                  TABLE 3                                                         ______________________________________                                        Compositions of samples and working and heat treatment                        conditions                                                                            Composition                                                                   (at %)      Working and heat treatment                                ______________________________________                                        Example                                                                              3      Ti-50Al-0.1TiB.sub.2                                                                        Twin-roll casting of                                                          sheet + HIP                                              4      Ti-50Al-1TiB.sub.2                                                                          Twin-roll casting of                                                          sheet + HIP                                              5      Ti-47Al-3Cr-1TiB.sub.2                                                                      Twin-roll casting of                                                          sheet + HIP                                       Comp. Ex.                                                                            6      Ti-50Al-0.1TiB.sub.2                                                                        High-frequency melting +                                                      isothermal forging                                       7      Ti-50Al-1TiB.sub.2                                                                          High-frequency melting +                                                      isothermal forging                                       8      Ti-47Al-3Cr   Plasma melting + annealing                               9      Ti-47Al-3Cr   Plasma melting + isothermal                                                   forging                                                  10     Ti-50Al       Plasma melting + isothermal                                                   forging                                                  11     Co-base       Commercially available                                          superalloy    material                                                 12     Ni-base       Commercially available                                          superalloy    material                                                 13     Ti-50Al       Twin-roll casting of                                                          sheet + HIP                                       ______________________________________                                    

The influence of the addition of TiB₂ on the refinement of the structurewas observed. Optical photomicrographs of metallographic structures insection in the thicknesswise direction are shown in FIG. 8 for acomparative material with TiB₂ not added thereto (Comparative Example10), in FIG. 9 for a material of the present invention with 0.1 at %TiB₂ added thereto (Example 3), and in FIG. 10 for a material of thepresent invention with 1 at % TiB₂ added thereto (Example 4).

All the photomicrographs shown in FIGS. 8 to 10 are in sets of five. Thephotomicrograph (1) located on the leftmost side is one showing thewhole section, the photomicrograph (2) located in the center top portionis an enlarged photomicrograph of the surface of the sheet in thethicknesswise direction thereof, the photomicrograph (3) located on thecenter bottom is an enlarged photomicrograph in the center of the sheetin the thicknesswise direction thereof, the photomicrograph (4) locatedon the right top is an enlarged photomicrograph of (2), and thephotomicrograph (5) located on the right bottom is an enlargedphotomicrograph of (3).

As can be seen from FIGS. 8 to 10, the addition of TiB₂ results inmarked refinement of the structure, and the addition of 1 at % TiB₂brings the grain diameter to a level of 10 μm. For the 300 ppm oxygenmaterial (50 at % Ti-50 at % Al) with TiB₂ not added thereto, which hadnot been rapidly cooled by the twin roll, the grain was coarsened to alarge diameter of 2 mm.

The influence of the addition of TiB₂ upon mechanical properties wasexamined. For the comparative material with TiB₂ not added thereto(Comparative Example 13), the material of the present invention with 0.1at % TiB₂ added thereto (Example 3), and the material of the presentinvention with 1 at % TiB₂ added thereto (Example 4), the tensilestrength at high temperatures is shown in FIG. 11, and the ductility isshown in FIG. 12.

Further, for the material of the present invention (Example 4), thetensile strength properties at room temperature and those at hightemperatures are tabulated in Table 4. For comparison, the tensilestrength properties at room temperature and those at high temperaturesfor the direct cast TiAl material with TiB₂ not added thereto(Comparative Example 13) are tabulated in Table 5.

                  TABLE 4                                                         ______________________________________                                        Tensile strength properties of sheet of                                       Ti-50Al-1TiB.sub.2 (Example 4)                                                          Yield        Tensile                                                Test temp.                                                                              stress       strength                                                                              Ductility                                      (° C.)                                                                           (MPa)        (MPa)   (%)                                            ______________________________________                                        25        450          590     2.1                                            800       384          540     7.4                                            1000      260          340     31.4                                           1100      165          197     47.2                                           ______________________________________                                    

                  TABLE 5                                                         ______________________________________                                        Tensile strength properties of sheet of                                       Ti-50Al (Comparative Example 13)                                                        Yield        Tensile                                                Test temp.                                                                              stress       strength                                                                              Ductility                                      (° C.)                                                                           (MPa)        (MPa)   (%)                                            ______________________________________                                        25        --           348     0                                              800       369          417     0.7                                            1000      229          346     4.8                                            1100      179          221     20.6                                           ______________________________________                                    

As can be seen from the results, for the material of the presentinvention, the addition of TiB₂ could increased the high-temperatureductility without sacrificing the high temperature strength and, inaddition, improved the ductility at room temperature from 0% to 2.12%.

Next, the tensile properties at high temperatures of a TiAlintermetallic compound-base alloy produced by the process of the presentinvention were compared with those of that produced by the conventionalprocess. The process according to the present invention was performedunder conditions of twin roll casting and oxygen concentration 2500 ppm,while the conventional process was performed under VIM melt process,isothermal forging, and oxygen concentration 1000 ppm.

For the thin sheet of Ti-50Al-0.1TiB₂ produced by the process of thepresent invention (Example 3) and that produced by the conventionalprocess, the relationship between the yield stress and the temperatureis shown in FIG. 13. Further, for the thin sheet of Ti-50Al-1TiB₂produced by the process of the present invention (Example 4) and thatproduced by the conventional process (Comparative Example 7), therelationship between the yield stress and the temperature is shown inFIG. 14.

As can be seen from the results shown in FIGS. 13 and 14, the TiAlintermetallic compound-base material produced by the process of thepresent invention had markedly improved strength at high temperatures.

Further, as is apparent from the comparison of the properties of thematerial of the present invention with the conventional TiAl materialshown in FIG. 15 and the comparison of the properties of the material ofthe present invention with those of the superalloys shown in FIG. 16,the properties of the material of the present invention are muchsuperior to those of the conventional high-temperature strength alloys.Further, the specific gravity of the material of the present inventionis low and 3.8 which is comparable to that of ceramics, offering highspecific strength.

COMPARATIVE EXAMPLES 6 to 13

For comparison, conventional TiAl alloy materials as shown in Table 3,that is, a material having a composition outside the scope of theinvention, a material produced by a process other than the process ofthe present invention, and a commercially available material, wereprovided.

EXAMPLE 6

An aluminum metal and a sponge titanium were mixed together to give acomposition of Ti: 50 at % and Al: 50 at %, and the mixture was meltedby high-frequency melting (VIM), whereby a master alloy was prepared bya melt process. In this case, in the final stage of pouring of themolten metal, a TiB₂ powder, in an amount corresponding to 1 at %,wrapped in an Al foil was added to the molten metal, thereby dispersingthe TiB₂ powder in the γTiAl matrix.

The ingot thus obtained was cut into a small piece of about 10×10×10 mmwhich, together with agate, was placed in a ball mill. The interior ofthe vessel was once evacuated, and the atmosphere in the interior of thevessel was replaced with Ar, followed ball milling for 24 hr.

The resultant TiAl powder was placed in a stainless steel vessel whichwas then covered and evacuated. Then, the boundary between the cover andthe vessel was subjected to electron beam welding. The TiAl wrapped inthe vessel was pressed with a reduction ratio of 80% at a hightemperature of 1200° C. by means of a hot press to prepare a sheet.

Thus, fine alumina (Al₂ O₃) was dispersed in an oxygen concentration inthe range of from 1000 to 5000 wt ppm and in a size of particlediameters ranging from 200 to 500 nm. Further, an intermetalliccompound-base alloy material comprising TiAl (Ti: 50 to 53 at %, Al: 47to 50 at %) containing an additive element (at least one of Cr, Mn, andV: 1 to 3 at %) with a boride having a diameter of not more than 500 nmdispersed in a B concentration of 0.1 to 10 at % was prepared.

EXAMPLE 7

An aluminum metal and a sponge titanium were mixed together to give acomposition of Ti: 50 at % and Al: 50 at %, and the mixture was meltedby high-frequency melting (VIM), whereby a master alloy was prepared bya melt process. In this case, in the final stage of pouring of themolten metal, a TiB₂ powder, in an amount corresponding to 1 at %,wrapped in an Al foil was added to the molten metal, thereby dispersingthe TiB₂ powder in the γTiAl matrix.

The ingot thus obtained was subjected to electrical discharge machiningto prepare a cylinder, having a size of 600φ×600 mm, which was thenhot-pressed with a reduction ratio of 80% under a vacuum of 10⁻⁶ Torr ata temperature of 1200° C. and a strain rate of 5×10⁻⁴ sec⁻¹.

Thus, fine alumina (Al₂ O₃) was dispersed in an oxygen concentration inthe range of from 1000 to 5000 wt ppm and in a size of particlediameters ranging from 200 to 500 nm. Further, an intermetalliccompound-base alloy material comprising TiAl (Ti: 50 to 53 at %, Al: 47to 50 at %) containing an additive element (at least one of Cr, Mn, andV: 1 to 3 at %) with a boride having a diameter of not more than 500 nmdispersed in a B concentration of 0.1 to 10 at % was prepared.

EXAMPLE 8

The TiAl-base intermetallic compound sheets prepared in Examples 3, 4and 5 were subjected to high-temperature pack rolling with a reductionratio of 50%, wherein an alumina foil was used as a separating materialand packing was carried out using a Ti alloy, under conditions of vacuum10⁻⁶ Torr, 1200° C. and strain rate 5×10⁻⁴ sec⁻¹. Thus, pack rolling wascarried out.

As indicated in each of the above examples, according to the presentinvention, the resultant cast strips or treated sheets had markedlyimproved mechanical properties. This improvement is considered to derivefrom the fact that a reduction in inclusion of impurities resulted in areduction in oxygen concentration of the material, offering an optimaloxygen content. Further, agitation by high frequency finely divided theoxide, and this state could be frozen by direct twin-roll casting.Further, the formation of an even fine structure by alumina/boridecombined precipitation effect could offer a ductility of 2.12% at roomtemperature while maintaining the strength at high temperatures.

Industrial Applicability

TiAl intermetallic compound-base alloy materials produced according tothe present invention have excellent tensile strength at hightemperatures and ductility at high temperatures and room temperatureand, hence, can be utilized in exhaust valves for automobiles,turbo-chargers, and turbine blades of engines for jet airplanes.

What is claimed is:
 1. A process for producing an intermetallic compoundbase alloy material having excellent room-temperature strength,room-temperature ductility, high-temperature strength, andhigh-temperature ductility, characterized in that fine TiB₂ having adiameter of not more than 500 nm is previously dispersed in thepreparation of a master alloy by a melt process, a calcia crucible or analumina (Al₂ O₃) crucible coated with a calcia (CaO) powder is used as acrucible in high-frequency melting in the melting of the master alloy, aTi plate is heated to 800 to 1100° C. in a VIM (Vacuum InductionMelting) vessel to conduct gettering of oxygen present within thevessel, thereby lowering the concentration of oxygen in the atmosphereof the vessel to not more than 0.2%, and casting is carried out in thisstate to produce an ingot which is then subjected to isothermal forgingto bring the structure to a fine grain structure.
 2. A process forproducing a TiAl foil, the in that a sheet of TiAl produced by a processaccording to claim 1 is molded by high-temperature pack rolling whereinan alumina foil or a calcia powder is used as a release material andpacking is carried out using a Ti alloy or stainless steel.
 3. A processfor producing an intermetallic compound-base alloy material wherein finetitanium boride (TiB₂) particles having a diameter of not more than 500nm are previously disposed in a master alloy during preparation of themaster alloy by a melt process, a calcia (CaO) crucible or an alumina(Al₂ O₃) crucible coated with a calcia powder is used as a crucible forhigh-frequency melting of the master alloy in a vacuum induction melting(VIM) vessel, a Ti plate is heated to 800 to 1100° C. in the vessel toconduct gettering of oxygen present within the vessel and thereby lowerthe concentration of oxygen in the atmosphere of the vessel to not morethan 0.2%, and melting of the master alloy and casting of the meltedmaster alloy is carried out in said atmosphere to produce a casting ofthe alloy material.
 4. A process according to claim 3, wherein saidcasting is a sheet of the alloy material and the sheet is molded toproduce a TiAl foil by high-temperature pack rolling in which analuimina foil or a calcia powder is used as a release material andpacking is carried out using a Ti alloy or stainless steel.
 5. A processaccording to claim 3, wherein said casting is an ingot of the alloymaterial and the ingot is subjected to isothermal forging to produce afine grain structure.
 6. A process according to claim 3, wherein thecomposition of the master alloy is such that the casting comprises finealumina (Al₂ O₃) particles having a particle diameter of 200 to 500 mmand dispersed at intervals of not more than 10 μm in the matrix to givean oxygen concentration of 1000 to 5000 ppm by weight, titanium boride(TiB₂) particles dispersed to give a boron concentration of 0.1 to 10 at%, and TiAl having a Ti content of 50 to 53 at % and an Al content of 47to 50 at %.